The present invention relates to heat-resistant materials. Example of such materials are ceramic and metal-ceramic materials, generally known as cermets, heat-resistant metals and alloys and also refractory concrete. The invention can be advantageously used for all of the above-mentioned materials.
Widely known in the art are refractory ceramic materials based on refractory oxides of metals. Among these are differentiated materials with a monolithic structure and with a porous structure.
Refractory ceramic materials possessing a porous structure have a higher heat resistance as compared to the same materials of a monolithic structure.
The higher heat resistance of the former can obviously be explained by the fact that pores retard the development of microcracks resulting from thermal stresses in the material body. However, a porous structure reduces the mechanical strength of the material and hence, its resistance to erosion. Due to this, porous ceramic materials have but limited application. In particular, they may be used only under conditions of a gaseous medium not containing fumes of alkali metal compounds.
This is attributed to the fact that the condensing of alkali compounds in the pores of a refractory material leads to further hydration and carbonization of the alkaline compounds which results in a sharp increase in the volume of the material and thereby in its deterioration. The utilization of porous ceramic materials in metal or glass melting furnaces is not justified since the refractories are exposed to a purely mechanical effect or an aggressive chemical action of the molten metal or glass.
Moreover, these materials can not be used in high temperature assemblies of a magnetic hydrodynamic generator whose medium contains fumes of an alkali metal. Condensation of these fumes occurs within a zone confined between 900.degree. C and 1200.degree. C isotherms. On cooling, alkali compounds become hydrated and carbonized by moisture and carbon dioxide from the air, respectively, which is accompanied by a sharp increase in the material volume and as a result by refractory peeling off or exfoliation.
There are also well known refractory ceramic materials based on refractory oxides of metals with fillers from refractory oxides of metals, the crystalline structure of these ceramic materials representing strand- or needle-like monocrystals.
These materials possess a higher mechanical strength and greater resistance to erosion as compared with porous materials.
The employment of such materials, however, involves considerable difficulties. The main problem is to ensure and maintain orientation and integrity of the above-mentioned crystals on their introduction into the matrix which, in this particular case, provides for the material as a whole to obtain a higher mechanical strength and improved resistance to erosion.
At present, there are no sufficiently reliable methods of shaping these materials. Even the most practicable method of shaping which is slip casting, does not provide the above-mentioned orientation and integrity of strand- or needle-like monocrystals. Up to now such materials have been produced, in fact, only under laboratory conditions.
Moreover, the process itself for producing strand-or needle-like crystals is at a stage of laboratory studies; their commercial production is not yet under way. Obviously, the manufacture of refractories with the afore-mentioned fillers is an expensive and inefficient process, therefore at the present time it is difficult to talk about their commerical application.
Well known cermets have a disadvantage consisting in that when used under oxidizing conditions or exposed to the effect of a high speed and high temperature gas flow (for instance, in the channel of a magnetic hydrodynamic generator) their resistance to erosion proves to be insufficient.
As experience has shown, these cermets can withstand no longer than 100 hours of operation under such conditions.
As known, at high temperatures heat-resistant materials start creeping and, in addition to this, have a low resistance to oxidation and erosion.
Well known types of refractory concrete contain refractory oxides of metals as filler materials, such as corundum or periclase in the form of polycrystalline fines obtained by grinding sintered briquettes or electric melted ingots.
The main disadvantage with the above types of concrete is that under the effect of a high temperature oxidizing gas flow with an alkali metal additive (as, for example, in the channel of an open-cycle magnetic hydrodynamic generator), rapid deterioration of the concrete surfaces occurs due to erosion and reaction between the aggregate (in the case of corundum) and the alkali metal.